1. Field of the Invention
The present invention relates to an illumination optical system, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
Recently, there has increased a demand for further miniaturization of semiconductor devices. The K1 coefficient (=resolution line width×numerical aperture of projection optical system/wavelength), which is called resolution latitude, has decreased beyond 0.35 and now reaches a level of 0.3 or less. Many proposals for more effectively utilizing the photolithography have been made to meet such a demand. In other words, Super-High Resolution Technology also called RET (Resolution Enhancement Technology) has been proposed. One example of the RET is to apply an auxiliary pattern and/or a line width offset to a reticle pattern depending on optical characteristics of an exposure optical system.
Another example capable of providing an advantage comparable to or better than that obtained with optimization of a reticle is a method of optimizing the shape of an effective light source of an illumination optical system depending on the reticle pattern. Such a method is generally called Modified Illumination Method (or Super-High Resolution by Illumination Control). The term “effective light source” means a light intensity distribution in the pupil plane of an illumination optical system or an angle distribution of light incident upon a target surface to be illuminated (i.e., a reticle plane).
As the Modified Illumination Method, multipole illumination using a plurality of poles, such as a dipole or a quadrupole, has recently become more important. Particularly, the multipole illumination is often practiced in a shape obtained by cutting a desired angular range from an annular distribution. An annular ratio and an opening angle are used as parameters of the multipole illumination in many cases. The annular ratio is defined as a ratio of an inner radius/an outer radius of the effective light source distribution. The opening angle is defined as an angle formed by each pole relative to the center of a light beam, i.e., an optical axis.
As one example for producing the multipole modified illumination, there is known a method of arranging a prism having a concave refractive surface and a prism having a convex refractive surface on the side closer to a light source than an integrator (which is disposed in a conjugate plane with respect to the pupil of a projection optical system) in an illumination system, and of zoom-moving the prisms. With the zoom-moving of the prisms, the annular ratio and the opening angle of an effective light source can be adjusted. Further, a zoom lens having a variable focal length is arranged downstream of the prisms to enlarge and reduce a light intensity distribution formed on the integrator.
For example, Japanese Patent Laid-Open No. 2003-297727 describes a method of arranging, downstream of a first prism unit comprising a pair of concave and convex surfaces with complementary refractive powers, a second prism unit comprising a pair of concave and convex surfaces with complementary refractive powers. Herein, the term “complementary” means that refractive indices are substantially the same, and that the convex surface and the concave surface have substantially the same shape in match with each other.
Also, Japanese Patent Laid-Open No. 2004-63988 describes a method of producing multipole illumination by forming an annular distribution with a conical refractive surface, and by arranging a light blocking member for angle restriction downstream of the conical refractive surface.
Meanwhile, it has recently been demanded by device makers to change, in match with a reticle pattern, the annular ratio while keeping the opening angle constant, or the opening angle while keeping the annular ratio constant. The annular ratio can be changed while keeping the opening angle constant by the known method of arranging the light blocking member for angle restriction downstream of the conical refractive surface and zoom-moving the conical refractive surface.
With the known method, however, a large loss in quantity of light is resulted because the light blocking member absorbs the light. On the other hand, if the pair of concave and convex refractive surfaces with complementary refracting actions is just zoom-moved without using the light blocking member, a problem arises in that the annular ratio and the opening angle are both changed.